Metals are susceptible to corrosion. Atmospheric corrosion is of particular concern. For example, when polymer coatings such as paints or sealants are applied to the metal, corrosion of the underlying metal may cause a loss of adhesion between the polymer coating and the base metal. A loss of adhesion between the coating and the base metal may similarly lead to corrosion of the metal. Aluminum alloys frequently require corrosion protection and improvements in adhesions between the base aluminum and subsequent coatings.
Generally, corrosion processes describe the oxidation of metal at its surface which acts to weaken and/or disfigure the metal. Most metals are active enough to be converted to their oxides, and it is generally accepted that corrosion occurs by electrochemical action involving the creation of small galvanic cells on the surface of the metal. More specifically, this invention is directed to compositions comprising corrosion inhibitors based on polycarboxylic anions and a variety of cations. The inhibitors are designed to be added to film forming or other compositions to reduce the corrosion of the metal substrate on which the materials are applied. Individual polycarboxylate compounds show corrosion inhibition for selected metals like steel or aluminum, in certain accelerated corrosion tests, but none are effective for multiple accelerated corrosion tests. It was discovered that specific combinations of polycarboxylate metal salts provided synergistic corrosion inhibition that would not be predicted by the performance of the individual salts. The novel feature of this invention is the combination of multiple metal polycarboxylate compounds, with the same or varying cations, to provide superior corrosion resistance compared to the individual carboxylate compounds. In addition, the corrosion resistances of coatings using active aluminum alloy pigments are enhanced by the synergistic combination of two or more polycarboxylate metal salts.
The prior art demonstrates corrosion inhibition by individual mono- and polycarboxylate compounds as additives to protective coatings but their performance is limited. None of the prior art predicts the synergistic effects obtained by two or more specific metal polycarboxylates. For example, compositions based on hexavalent chromium, like zinc chromate, barium chromate and strontium chromate, are superior corrosion inhibitors and have been used for approximately 100 years to protect aircraft and other valuable assets which would otherwise corrode more quickly in the environment. Protective primers used in naval aviation, according to the materials specifications MIL-PRF-85582, MIL-PRF-23377 and TT-P-2760, describe and qualify coatings based on chromate inhibitors. Although chromate-based inhibitors are technically excellent, the hexavalent chromium species is a known carcinogen and has been targeted for replacement since the early 1970's. Corrosion inhibitors based on non-chromate compounds have been implemented but are to date less effective for protecting various metals such as steel, aluminum and its alloys.